For achieving high performance, low temperature manufacturing process and low cost of a thin film transistor (TFT), a wide variety of channel layer materials have been studied at present. The materials include amorphous silicon, polycrystal silicon, microcrystal silicon and organic semiconductor, for example.
An oxide semiconductor found in recent years is another potential candidate as the material. A TFT using in its channel layer an amorphous In—Zn—O(IZO) thin film and an amorphous In—Ga—Zn—O(IGZO) thin film is disclosed in papers: Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006); and Yabuta et al., Appl. Phys. Lett., 89, 112123 (2006).
A TFT exhibits a threshold voltage different according to a semiconductor material of its channel layer and manufacturing process. The threshold voltage of the TFT is changed by various causes (such as manufacturing process hysteresis, aging, electrical stress and thermal stress). The electrical stress is caused by applying voltage or current to a semiconductor. The thermal stress is caused by external heating or Joule heat generated by applying current to a semiconductor. Actually, these stresses are sometimes simultaneously applied to the TFT.
The threshold voltage of the above oxide semiconductor TFT is unexceptionally changed by the electrical stress or a composite of the electrical and the thermal stress. This is disclosed in papers: Riedl et al, Phys. Stat. Sol., 1, 175 (2007); and Kim et al., International Electron Device meeting 2006 (IEDM'06), 11-13, (2006).
There is disclosed that irradiating the oxide semiconductor TFT with a visible light and an ultraviolet ray changes various properties of the TFT including the threshold voltage in papers: Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006); and Gorrn et al., Appl. Phys. Lett., 91, 193504 (2007). For polycrystal silicon, Japanese Patent Application Laid-Open No. H10-209460 discloses a method of reducing the threshold voltage by irradiating the channel layer of the TFT with light.
Many experiments have been done to provide each pixel with an electrical circuit formed of a plurality of transistors and capacitors in a light emitting display apparatus to compensate change in the threshold voltage.